Thermal bridge

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A thermal bridge (often colloquially referred to as a cold bridge ) is an area in components of a building that conducts heat better and thus transports heat to the outside faster than it passes through the adjacent components. As a result, the corresponding component (e.g. a corner of the room) cools down more quickly and experiences a lower temperature than the surroundings. If the temperature falls below the dew point , the moisture contained in the room air condenses on the component.

Due to the increased moisture in the material, molds tend to form on thermal bridges .


Thermal bridges in a lattice window with condensation or ice flowers

The increased heat conduction in thermal bridges causes heat to flow away there. This results in a lower internal surface temperature at this point and there is a risk of mold formation and thus health hazards. Further consequences are the risk of condensation and damage to the building structure.

  • Increased heating demand: Thermal bridges lead to higher transmission heat loss and thus to a higher need for post-heating and higher heating costs.
  • Condensation failure : In the area of ​​thermal bridges, the room-side surface temperature of components drops more sharply than in the undisturbed areas at low outside temperatures. When falling below the dew point falls condensate ( condensation ) from the surface.
  • Mold : There is a risk of mold forming on thermal bridges. This does not only occur in the event of condensation, but already at a relative humidity of 80% on the component surface (due to the surface temperature) (some molds already at 70%).

to form

A distinction is made between material-related and geometric thermal bridges.

  • Material-related thermal bridges arise from the materials used. Since every material conducts heat differently, building materials with high thermal conductivity are particularly responsible for the formation of thermal bridges. These are especially metals that are generally very good conductors of heat.
  • Geometric thermal bridges arise when the inner surface is not the same as the outer surface. In general, the more compact a building is, i.e. the smaller the ratio of external surface to internal surface, the lower the energy losses. The cause is the cooling fin effect, which occurs , for example, on the outside corners of houses, dormers and bay windows .
Geometric thermal bridge: Section through the corner of an externally insulated external wall with isotherms drawn in at an external temperature of –10 ° C and an internal temperature of 20 ° C. The isotherm of 18 ° C is near the corner on the wall surface, but away from the corner inside the wall
Constructive and geometric thermal bridge

Constructive thermal bridges are often mentioned as a further category:

Constructive thermal bridges are understood to be thermal bridges that result from structural constraints. The following component connections typically penetrate the insulation level :

Further thermal bridges can be found in the following components:

  • Roller shutter boxes
  • Wall soles
  • Window frames and lintels
  • Radiator fixings in the masonry
  • Radiator niches
  • Ceiling connections
  • Corners in the house (see graphic on the right)

These components usually have an increased heat transfer coefficient .


With the first thermal insulation ordinance of 1977, the first requirements for the thermal insulation of buildings were made. The minimization of the transmission heat loss in the area of ​​thermal bridges has long been a recognized rule in structural engineering . For example, wood wool lightweight panels ( Heraklith insulation panels) were installed in the area of ​​radiator niches . At that time, the primary goal of these measures was to avoid mold formation (saving on heating costs was at best a secondary goal). Even today, the requirements for hygienic heat protection must be complied with and must be verified in accordance with DIN 4108-2 . A minimum surface temperature of 12.6 ° C must be maintained in order to avoid condensation and mold formation. Further tightening was introduced by the Energy Saving Ordinance in several amendments. The minimum requirements for heat losses from thermal bridges are regulated here.

According to the Energy Saving Ordinance, there are three methods to consider thermal bridges energetically in the transmission heat losses:

  • Simple method: With this calculation, the thermal bridges in the building are not verified. For this, a thermal bridge surcharge in the form of an increase in the mean U-value must be added to the total heat loss of the building. For external insulation this is ΔU WB = 0.1 W / (m 2 · K).
  • Simplified method: The thermal bridge calculation can also be carried out in accordance with supplement 2 to DIN 4108. In this document published by Beuth Verlag , examples of professional designs of thermal bridges from the new building area are presented, all of which comply with the minimum thermal insulation . In this case, an improved thermal bridge allowance (ΔU WB = 0.05 W / (m 2 · K)) can be applied.
  • Detailed method: Here, the thermal bridge is precisely verified using the appropriate software. The actual heat losses are thus taken into account. By examining the thermal bridges in detail, their execution is particularly important. In this case, the heat transfer coefficient (ψ value or also called psi value) is determined individually for a thermal bridge.

Different meaning

A thermal bridge of a different meaning is mentioned in Warming device (kitchen device) .

See also


  • Chapter 10 Thermal Bridges. In: RWE Bau-Handbuch , 15th edition from 2015
  • Frank Frössel: Mold in apartments. When the mushroom lives to sublet. Baulino, Waldshut-Tiengen 2006, ISBN 3-938537-18-3 .
  • Walter Heindl (Ed.): Thermal bridges. Basics, simple formulas, heat losses, condensation, 100 calculated building details , Springer, Vienna 1987, ISBN 3-211-82024-8 .
  • Peter Häupl (Ed.): Textbook of building physics: Sound - heat - humidity - light - fire - climate . 7th, completely revised and updated edition, Springer Vieweg, Wiesbaden 2013, ISBN 978-3-8348-1415-9 .
  • K. Siegele: Assessment and calculation of thermal bridges. In: Deutsche Bauzeitschrift No. 9/2014, pp. 74–77

Web links

Commons : thermal bridges  - collection of images, videos and audio files

Individual evidence

  1. Klaus W. Usemann: Building technology: Lexicon of terms . Oldenbourg-Industrieverlag, Munich 2001, ISBN 3-486-26395-1 , p. 265 ( full text in Google Book Search).
  2. ^ Karl-Friedrich Moersch: The new energy pass from AZ . Walhalla Fachverlag, Regensburg 2008, ISBN 3-8029-3568-3 , p. 78, 123 ( full text in Google Book Search).
  3. Requirements for the thermal protection of thermal bridges
  4. Overview of possible thermal bridges
  5. EneV2009 Annex 1, Section 2.3; EneV Annex 3 Section 8.1; DIN V 4108-6: 2003-06 section
  6. The heat transfer coefficients ψ and χ
  7. Calculation of the heat transfer coefficient ψ